The present invention relates to a pneumatic tool and more particularly, to a speed adjusting device of a pneumatic grinder and which stabilizes the rotational speed of the grinder and makes sure that the speed will not be fluctuated by changes of the pressure, and the speed adjusting device will not malfunctioned
A conventional pneumatic grinding tool such as grinders or polishers is designated by reference 10 in FIG. 1. A rotator 14 is installed in a cylinder 13 in a casing 12 and the rotator 14 has a grinding plate 16 on the axle 15 thereof. A speed adjusting device 20 is installed in the chamber 17 in the casing 12 and the speed adjusting device 20 is co-rotated with the rotator 14. After the high pressure air is introduced in the pneumatic tool, the air pressure enters in the chamber 17 via an inlet 18 and then enters in the cylinder 13 via several holes 19 to drive the rotator 14. By the rotation of the rotator 14 to grind objects by the grinding plate 16.
Due to many factors affect the air pressure so that the air pressure for the pneumatic tool is not constant. When the air pressure raised, the rotational speed of the rotator 14 increases. On the contrary, the speed reduces when the air pressure drops. Because the grinding plate 16 has a pre-determined speed limit, it could be broken when the speed is higher than the pre-determined speed limit. Therefore, a speed adjusting device 20 is required to prevent the situation mentioned above.
As shown in FIGS. 2, 3 and 4, a conventional speed adjusting device 20 has an axle 22 and an adjusting rod 24 which is a straight rod and fixed to the axle 22 and rotatable with the axle 22. A stop 26 is slidably mounted to an end of the adjusting rod 24. The stop 26 is biased by a spring 27. Two adjusting members 28 are respectively engaged with the recesses 241 in two ends of the adjusting rod 24. Each adjusting member 28 has a tip end 281 at its pivot able end. The axle 22 is fixed to the rotator 14 at its front end and is rotatable with the rotator. The stop 26 is located in front of the inlet 18 as shown in FIG. 1.
When the rotator 14 is not rotated, the stop 26 is pushed by the spring 27 and maintained to slide forward, and the two adjusting members 28 are not open outward as shown in FIG. 2. When the rotator 14 is rotated, the speed adjusting device 20 is rotated and the two adjusting members 28 are moved outward because of the centrifugal force as shown in FIG. 4. The stop 26 is pushed backward by the tip end 281 of the speed adjusting members 28 so that the distance between the stop 26 and the inlet 18 is changed. The outward movements of the two adjusting members 28 are increased when the air pressure and the speed of the rotator 14 are high. This will move the stop 26 toward the inlet 18 to reduce the volume of the air coming in to prevent the rotator 14 from over-speed.
There are some inherent shortcomings:
1. As shown in FIG. 3, the speed adjusting device 20 is an elongated shape so that there are cut points xe2x80x9cbxe2x80x9d on the outside of the two longitudinal sides xe2x80x9cAxe2x80x9d. The cut points xe2x80x9cbxe2x80x9d generate turbulent air flow and make the speed adjusting device to be in an idle status which makes the tool shaking.
2. In the situation in FIG. 4, the tip end 281 of the adjusting members 28 pushes the stop 26 due to the centrifugal force, and the direction of the stop 26 is parallel with the direction of the air flow. The inlet air directly affects the stop 26 so that the adjusting members 28 bear the inlet pressure. The tip end 281 is then worn out and becomes too short to move the stop 26 such that the function of adjusting the speed of the rotator 14 is loosened. The adjusting members 28 may also shift abnormally as shown in FIG. 5 and jammed with the stop 26. This could mis-function the tool.
3. The adjusting rod 24 is a long rod and the two recesses 241 on two ends of the rod 24 makes the adjusting rod 24 to be a hollow rod which tends to be deformed when the rod is proceeded with heat treatment. The two ends of the rod are symmetrically deformed and result shaking when the rod is rotated. Similarly, the stop 26 is a thin member which is deformed after being heat-treated so that the stop 26 could have interference friction with the axle 22.
4. The speed adjusting device 20 bears the air pressure directly so that the parts of the device tend to be worn out by the air pressure.
5. Because the stop 26 is affected by the high pressure air flow, it is difficult to move back when the pressure increased and the adjusting member 28 are difficult to open outward. When the inlet pressure is higher than 90 psi as shown in FIG. 6, the function of the speed adjusting device reduced and the speed of the rotator increases so that the conventional speed adjusting device cannot control the speed of the rotator.
The primary object of the present invention is to provide a speed adjusting device for a pneumatic grinding tool and which effectively controls the speed of the rotator of the pneumatic grinding tool.
Another object of the present invention provides a speed adjusting device for a pneumatic grinding tool wherein the pressure that the speed adjusting device bears can be reduced.
The present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a preferred embodiment in accordance with the present invention.